The noise from a switching power supply mounted on electronic equipment is restricted by classes as represented by the FCCI. There are various causes of generating a noise in the power supply, and the noise is mainly generated around a semiconductor element which turns on/off a large amount of power. Especially, a high-frequency component propagates as the radiated noise through space and causes a malfunction of various types of electronic equipment. Therefore, a regulation value is set for each frequency band. The switching power supply is provided with an anti-noise measure for a semiconductor element, mainly a MOS-FET or a diode. As a typical example of the anti-noise measure for the MOS-FET or the diode, there is an anti-noise measure using a CR snubber or ferrite beads.
The anti-noise measure is selected depending on a balance of the effects, cost and mounting space. When the performance is especially taken into consideration, the anti-noise measure using a Co-based amorphous material is used mainly as described in Reference 1. Since the Co-based amorphous material has excellent magnetic characteristics, its noise reducing effect is better than the ferrite beads. But, a toroidal core using a Co-based amorphous magnetic ribbon is generally covered entirely with a resin, and therefore it has a problem that the resin penetrates between layers of the magnetic ribbon and applies a stress to the toroidal core by contracting when dried, resulting in degradation of the magnetic characteristics.
Meanwhile, Reference 2 and Reference 3 describe a noise suppression element that a core is inserted into a bottomed container, and a lid is fixed to house the core in the container. When the lidded container is used, the problem involved in the contraction of the resin is avoided, and the magnetic characteristics can be suppressed from degrading. But, the lidded container requires that its lid and container body are separately produced and fixed into one body by assembling them. To produce the lid and the container body with a resin material, it is necessary to prepare their independent metal molds and to perform resin molding by using the metal molds.
As described above, the lidded container requires the separate metal molds for the lid and the container body, and their preparation has a problem that the preparation percentage in the production cost is large. Besides, the noise suppression element described in Reference 2 requires a step of inserting the lid into the container body. The noise suppression element described in Reference 3 requires a step of fixing the container body and the lid by welding. Since the noise suppression element using the lidded container requires a step of attaching the lid, it has a problem that it is inferior in mass production.
Reference 1: JP-B2 2602843
Reference 2: JP-A Hei 11-345714 (KOKAI)
Reference 3: JP-A 2001-319814 (KOKAI)